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(P1) According to dinosaur fossils, dinosaurs had an oxygen isotope ratio in their bones that suggests that their CORES had roughly the same temperature as their LIMBS.
(P2) Today, cold-blooded animals have much warmer CORES than LIMBS.
(MC) Therefore, dinosaurs were probably warm-blooded.
Weaken
This argument assumes, among other things, that warm-blooded animals, unlike cold-blooded animals, do NOT have much warmer CORES than LIMBS, or some other temperature distribution that deviates even more from the dinosaurs'. To anticipate the right answer, I thus was expecting a weaking option targeting this assumption.
(A) Unlike cold-blooded animals, warm-blooded animals only have SLIGHTLY warmer CORES than LIMBS. This goes in the direction of my pre-phrase but is not very strong. Crucially, it remains more likely that dinosaurs were warm-blooded than that they were cold-blooded, just as the author claims. So this answer choice does not seem to actually weaken, even though it gets at the assumption that the author makes, and that I had identified as the weak point of their argument.
(B) Dinosaur fossils don't actually allow you to do the temperature inference described in (P1). This answer is very unusual in that it attacks a premise rather than the reasoning in the argument. Nevertheless, this answer choice definitely weakens, since it takes away the data about dinosaurs that the author presupposes. Keep this answer choice around but be vigilant; see if a less premise-focused answer choice is available.
(C) About oxygen generally. Does not seem to pertain to the argument.
(D) Body temperatures in small and large animals other than dinosaurs. Does not seem to connect directly to the argument; especially since the stimulus does not identify dinosaurs as either small or large.
(E) Warm-blooded animals are more active and use more oxygen than cold-blooded animals. This again does not seem to relate directly to the argument under consideration.
(C), (D), and (E) turn out to be largely unrelated to the argument in the stimulus, and (A) does not seem to weaken the inference made by the author. This leaves (B) as the only remaining answer choice, and thus (B) must be right.
Nevertheless, (B) feels very much uncomfortable and is unusual. (B) just straight up contradicts information that we get in the stimulus, rather than attacking the author's reasoning. It also seems unusual to have this sort of unexpected answer choice so early in the section; just expecting straightforward questions in (Q1)-(Q10) is too naive.
I originally chose (A) because I got too focused on my anticipation of how the right answer could look like, and thus I neglected (B). Nevertheless, a more careful examination of what (A) and (B) are actually saying would have allowed me to get this question right. I need to stay alert to the details of individual answers and compare them against each other; a more thorough examination between (A) and (B) would have allowed me to see that (A) does not in fact weaken and that (B)'s unusual character does not prevent it from being the right answer here. Read answer choices carefully, compare them against each other, and choose the one that has the fewest problems.
